Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and particularly to a technique which eliminates a periodic density variation occurring in a subscanning direction.
Description of the Related Art
Generally, in an image forming apparatus (such as a printer, a copying machine, and a facsimile) using an electrophotographic process technique, a uniformly charged photoconductor (for example, a photoconductor drum) is irradiated with (exposed to) light based on input image data, and thereby an electrostatic latent image is formed on a surface of the photoconductor. By supplying toner to the photoconductor on which the electrostatic latent image has been formed, the electrostatic latent image is visualized to form a toner image. The toner image is transferred to a sheet directly or indirectly through an intermediate transfer member, and subsequently heated and pressurized at a fixing unit, whereby an image is formed on the sheet.
An image forming apparatus includes, as a constituent for image formation, various rotating members such as a photoconductor and a developer holding member. It has been known that a periodic density variation is caused in an image in a subscanning direction thereof by rotation runout of these rotating members. For example, since a gap (development gap) between a photoconductor and a developer holding member is periodically varied based on rotation runout of the photoconductor or the developer holding member, an electric field intensity is periodically varied even if a constant developing bias is applied. As a result, a density variation occurs in an image with the same period as a rotation period of the photoconductor or the developer holding member. Hereinbelow, a periodic density variation occurring in an image in a subscanning direction thereof is referred to as “periodic density unevenness”.
In a conventional image forming apparatus, in order to offset periodic density unevenness, correction data is generated which is corresponding to rotational positions (phases obtained by using a home position as a reference) of a photoconductor, based on, for example, a density profile indicating periodic density unevenness. With this correction data, followings are corrected: image forming conditions such as exposure energy (exposure time or exposure output), a charge voltage, a developing bias voltage, rotations of a developer holding member (for example, a developing roller), and a density value (gradation value) of input image data (for example, JP 2007-140402 A).
The density profile is generated, for example, by forming a density-corrective patch image (such as a halftone image with halftone density) on a toner image holding member such as an intermediate transfer belt, and detecting the image density of the corrective patch image. At that time, the corrective patch image is formed such that a length in a subscanning direction thereof is adjusted to be longer than the longest period length (generally, period length of a photoconductor) of period lengths (length corresponding to a rotation period) of rotating members by which periodic density unevenness occurs. The corrective patch image is formed to be longer than a plurality of period lengths of a rotating member, and results of the image density detection are averaged. By doing so, a highly accurate density profile can be obtained.
In order to improve the accuracy of density correction, it is preferable to update the density profile periodically or at a predetermined timing such as when starting a print job. The reason being that the density profile varies in accordance with variations in developability and transferability caused by an environmental and temporal influence.
In JP 2011-170156 A, it is disclosed that in an image forming apparatus, a relationship between a development gap and writing sensitivity is grasped in advance, and image forming conditions such as an exposure amount are corrected according to a rotation position of a rotating member.
However, in a case where the length of the corrective patch image in the subscanning direction thereof is increased, or where the density profile is frequently updated, although the accuracy of density correction is improved, the following problems occur: a load on a cleaning unit is increased when removing the corrective patch image formed on the toner image holding member; consumption of toner required for density correction is increased; and a longer period of time is required for density correction, which results in a decrease in productivity.
In addition, the technique described in JP 2011-170156 A cannot be appropriately applied to a case where the periodicity of a development gap has varied, and there may be a case where image quality is decreased by correcting image forming conditions.